1. Field of the Invention
The invention generally relates to a semiconductor process, and more particularly to a photolithography process and a photomask structure that can improve the dimensional transfer of pattern images from a photomask onto a substrate in the photolithography process.
2. Description of the Related Art
In the manufacture of a semiconductor device, a photolithography process is commonly conducted to transfer or image specific patterns predefined on a photomask to a semiconductor wafer or substrate. For this purpose, a photosensitive material (also called photoresist) is initially formed on the substrate. The photoresist layer can be either a positive photoresist or negative photoresist layer depending on whether an assigned region is to be removed after development. Then, the photoresist layer is exposed through the photomask to specific light radiation, so that the pattern defined on the photomask is imaged on the photoresist layer. The radiation used during exposure can include deep ultraviolet (DUV), X rays, electronic beams, ion beams or the like. After the exposure is completed, the substrate is processed to develop the transferred pattern and thereby form a patterned photoresist layer through which the substrate is etched to form the desired circuit pattern.
Since the circuit patterns become increasingly smaller, a photolithography process of higher precision is required. By reducing the wavelength of the exposure light in the photolithography process, small active devices and transistors can be realized by establishing small critical dimension. The critical dimension of the circuit pattern is defined as the smallest pattern line width.
A photomask conventionally includes one or more circuit patterns. The photomask can be of positive or negative type depending on whether the pattern images formed on the photomask are opaque or not. The negative photomask is usually preferred because light scattering in the negative photomask is in a smaller amount and particles falling in opaque regions are less likely to be developed. When light strikes fine particles, the particles absorb light energy. A part of the light energy is absorbed and becomes an internal energy, while the other part of the light energy emerges out under the form of light scattering. Light scattering is one issue to be solved in optical exposure. During the exposure, the particles contained in the photomask scatter light so that the critical dimension of the photoresist pattern formed on the substrate is biased with from the dimension set in the photomask pattern.
FIG. 1 is a schematic view of a conventional photomask. A conventional photomask 100 can include a first pattern 110 and a second pattern 120 of different sizes and an opening area 130. The opening area 130 is located between the first pattern 110 and the second pattern 120 and surrounds the second pattern 120. In FIG. 1, the first pattern 110 and the second pattern area 120 exemplary include line-shaped patterns and have similar line widths 112, 122 and inter-line pitches 114, 124, but the first pattern 110 occupies an area larger than the second pattern 120. In other words, the first pattern 110 has higher pattern density than that of the second pattern 120, i.e. the light transmission rate is higher through the first pattern 110 than through the second pattern 120.
During the exposure, the photomask 100 is placed on a semiconductor substrate having a photoresist layer thereon, and a light radiation is projected through the photomask 100 onto the substrate. The opening area 130 may cause light scattering around the second pattern area 120 of the photomask 100, which may result in the accumulation of light energy in the underlying photoresist layer.
FIG. 2 is a cross-sectional view of a pattern image obtained by the conventional photolithography process. After development, first and second photoresist pattern images 210, 220 are formed on the semiconductor substrate 10 corresponding to the first and second pattern 110, 120 of the photomask 100 as illustrated in FIG. 1. As shown in FIG. 2, the photoresist pattern image 220 formed on the substrate 10 has dimensions biased from those of the corresponding second pattern 120 set in the photomask 100. Reference numeral 222′ indicates the target line width as set in the photomask, while reference numeral 222 indicates the actual line width obtained after development. As shown, the line width 222 of the photoresist pattern image 220 is smaller than the line width 212 of the first photoresist pattern image 210. Since the first and second patterns 110, 120 are dimensionally configured with a same line width in the photomask 100, the pattern image of the second pattern 120 thus has been dimensionally biased in the photolithography process.
Conventionally, the pattern of the photomask having a smaller pattern density is configured beforehand to compensate the critical dimension biases occurring when pattern images of different densities are formed. However, this preliminary compensation becomes difficult to achieve as the critical dimensions of the semiconductor devices are increasingly reduced.
Therefore, a need presently exists for a photolithography process that can prevent the dimensional biases of pattern images formed in a photolithography process.